Fuse device



Feb. 22, 1966 P. J. RAYNO FUSE DEVICE Filed June 22, 1961 United StatesPatent 3,236,976 FUSE DEVICE Paul J. Rayno, Hudson Falls, N.Y., assignorto General Electric Company, a corporation of New York Filed June 22,1961, Ser. No. 118,822 5 Claims. (Cl. 200-135) The present inventionrelates to an electrical fuse device, and more particularly to a fusedevice adapted for use in electrical capacitors.

The use of fuses in conjunction with capacitors for isolating one ormore defective capacitor units from other units is already known in theart. While it is often advantageous to locate the fuse inside the woundcapacitor roll, the known types of capacitor fuses have not provedsatisfactory when so located, due to variation in characteristics andunreliability of prior art fuses in such location. One cause, forexample, appears to be the variation in the tightness of the roll inwhich the fuse is incorporated, resulting in large changes in the fusecooling rates. It appears that known types of fuses are unduly sensitiveto changing or varied environment such as may be encountered inconventional capacitor designs and either must be isolated from it orthe environment conditions must be made constant. These alternativesoften are not feasible from a practical or economic standpoint in thecommercial manufacture of capacitors. Aside from the above disadvantagesof prior art fuses as applied specifically to capacitors, known types offuse construction are often unsatisfactory due to the relatively longperiod required to blow the fuse at any level above thegiven rating.Known fuses, moreover, have the disadvantage that the current requiredto effectively blow the fuse is often unduly in excess of the carrying(rated) current, that is, the maximum current which the fuse can sustainbefore it begins to melt. In other words, such fuses have too high aratio of blowing current to carrying current.

It is an object of the invention to provide an electrical fuse whichavoids the above and other disadvantages of prior art fuses.

It is another object of the invention to provide an electrical fusewhich hasa shortened blowing time, has an accurately predictable blowingcurrent, has a lower ratio of blowing current to carrying current thanprior art fuses, and is more reliable in operation than known types offuses.

It is a further object of the invention to provide a fuse device,especially one adapted for electrical capacitor application, havingreliable operation even under a wide range of environmental conditions,and which is compatible with conventional capacitor dielectricimpregnants.

Other objects and advantages will become apparent from the followingdescription and the appended claims.

With the above objects in view, the present invention relates to a fusedevice which comprises an elongated electrically conducting metal memberhaving a melting point of not less than about 600 C. divided into twoportions separated by a gap, and a fusible member having a melting pointnot higher than about 400 C. connected at its ends to the ends of theseparated conducting portions and conductively bridging the gaptherebetween, the cross-sectional area of the fusible member at each endthereof being not less than the cross-sectional area of the metal memberat the points of contact therewith.

In a preferred embodiment as used in capacitors, the fuse member iscovered by a sheet of electrically insulating material, such as kraftpaper or the equivalent, the insulating sheet having characteristics asmore fully described hereinafter. In another preferred embodiment, theconducting member portions are tapered toward the ends of the fusiblemember.

The invention will be better understood from the following descriptiontaken in conjunction with the accompanying drawing, in which:

FIGURE 1 is a fragmentary perspective view of a fuse device of thepresent invention;

FIGURE 2 is a similar view of another embodiment of the invention;

FIGURE 3 is a similar view of still another embodiment of the presentinvention;

FIGURE 4 is a view of a partially unwound capacitor roll section, withportions broken away, incorporating a fuse device of the type shown inFIG. 2.

FIGURE 5 is an enlarged exploded view of the fuse device shown in FIGURE4; and

FIGURE 6 is a view, partly broken away, of a capacitor assembly in whichthe fuse device of the invention may be embodied.

Referring now to the drawing, and particularly to FIG- URE 1, there isshown a fuse device constructed in accordance with the inventioncomprising an elongated electrical conductor 2 divided into portions 2aand 2b by a gap, and fuse link 3 bridging the gap. Fuse link 3 iselectrically connected to the adjacent ends of conductor portion 2a, 212by solder joints 3a, 3b or by any other suitable means which effectivelyelectrically connect the parts. In accordance with the principles of theinvention, fuse link 3 is preferably circular in cross-section, has amelting point not higherthan about 400 C., its crosssectional area ateach end thereof is not less than the cross-sectional area of conductor2 at the points of contact therewith and is preferably substantiallyequal thereto, and conductor 2 has a melting point not lower than about600 C. For optimum results, the length of fuse link 3 should be betweenabout 5 to 20 times its diameter, and within this range better resultsare generally obtained with shorter fuse links than longer ones, as willbe explained hereinafter. Usually, also, there is an advantage in usingshorter links for fuses to be used in air or free oil as compared tofuses used inside a capacitor roll.

Examples of low melting point fusible materials which may be used forfuse link 3 in practicing the invention are as follows:

Table] Composition: Melting point, C. 20% Bi% Sn 200 40% Pb60% Sn 18633% Bi-67% Sn 166 50% Bi50% Pb 160 50% Bi27% Pb-l3% Sn10% Cd 72 Pb 327100% Sn 232 100% Cd 321 Conductive materials of high melting pointsuitable for use as conductor 2 are as follows:

Table II Composition: Melting point, C. Copper 1083 Silver 960 Bronze900 Brass 900 Aluminum 660 The ends of fuse link 3 may be soldered toconductors 2a, 2!) at joints 3a, 3b by use of a solder material ofsomewhat lower melting point than fuse link 3 itself. Such soldering maybe achieved practically by initially coating fuse link 3 with such asolder material, as, for example, one composed of 33% bismuth and 67%tin which has a melting point of 166 C., and then soldering the parts bysuitable application of heat at the ends of link 3 while in contact withends of conductors 2a, 2b in accordance with known techniques. Thespecific solder material just described would be suitable for a fuselink melting at 186 C., and as will be understood, different solderalloys would be selected for use with different link alloys.

The principle of the present invention, in one of its aspects, departsfrom the prior art in that while the prior art finds it desirable toprovide for heat conduction in a fuse away from the center of thefusible element and toward the terminals or conductors adjoining thefuse, the present invention in contrast provides for primary loss ofheat from the fusible link through its surface to the ambient mediumrather than from the link to the associated conductor. The prior artteaches away from the use of shorter as compared to longer fuse linksbecause the shorter length of conventional fuses does not melt asquickly as the longer link under the same overload current conditions,the prior art concept being that with the same current, the rate of heatconduction to the terminals with a longer link is less than with ashorter link.

In the case of fuse devices of the present invention, however, theshorter the fuse link, the lower is the rate of heat loss from the linkto the terminals, which is contrary to conventional fusecharacteristics.

By virtue of the construction provided by this invention, the fuse has anumber of advantages over known fuse designs. For one thing, the rangeof temperature between the maximum carrying current and the blowingcurrent is reduced to a minimum, resulting in very reliable andaccurately predictable operation of the fuse. Moreover, the amount ofheat given off by the fuse and associated parts into the environment ismarkedly reduced and there is thereby avoided the consequent detrimentaleffects to the electrical device with which it is associated, as well asto the fuse itself.

The use of a low melting point fuse link in accordance with theinvention contributes to the above-mentioned advantages, in that thefuse element melts at temperatures considerably below that ofconventional high melting point fuse elements, and thus the amount ofheat produced at blowing temperatures is quite small. This diminishesthe effect of the surrounding environment which in the case ofconventional fuse constructions has led to unreliable fuse operation.

In general, in practicing the invention, it is preferable that theconductors with which the fuse is in series he of thin, flat, stripconfiguration, a form which provides maximum heat-dissipatingcharacteristics. For this component, copper and silver are preferredmaterials because of their excellent electrical and thermal conductingcharacteristics, and such conductors in strip form contribute to a lowheating fuse.

FIGURE 2 shows another embodiment of the fuse device wherein theconductor portions 4a, 4b are tapered toward fuse link 5 which issoldered or otherwise secured to the peaks of the tapered ends, theother structural characteristics being as described in connection withFIGURE 1. Such tapered construction has the advantage of retarding heatflow from link 5 to conductor portions 4a, 4b, and therebyproportionately increases the percentage of heat flow from the linksurface to the ambient medium as compared to the heat flow from link 5to conductors 4a, 4b. This result is desirable because when less heatflow occurs to the conductors 4a, 4b, there is less influence ofenvironmental conditions on the operation of the fuse and the fuseproduces proportionately less heat for any given rating.

The tapered portions of conductors 4a, 4b preferably taper back for adistance at least equal to the width of the conductors and should bereduced in cross-section at the point of junction with fuse link 5 tothe extent that resistance to electrical flow per unit of length at thenarrowest point should be a value substantially equal to the electricalresistance per unit of length of the fuse link. When thus proportioned,fuse link 5 at high energy levels will open at the two points ofjunction, and at low overload blowing currents will open at the centerof the fusible element.

FIGURE 3 shows another embodiment of the invention comprising a fusedevice having a fuse link 6 bridging round conductors 7a, 7b. While sucha configuration does not have the advantage of strip-shaped conductorsas explained above, the FIGURE 3 form, which otherwise has the featuresand characteristics of the previously described fuses, is satisfactoryfor operation in a variety of applications. 1

FIGURE 4 shows a fuse device of the present lIIVfiIl tion as applied toan electrical capacitor. As illustrated, the capacitor comprises arolled capacitor section 8 made up in conventional arrangement of woundalternate strips of metal foil 9 and 10 serving as electrodes withinterposed strips of dielectric material 11 and 12 such as kraft paper,all interwound into a compact roll. Each alternate dielectric layer maybe of a single sheet but is usually constituted by a plurality ofsheets, and has a greater width than the electrode strips 9, 10. Asshown, the dielectric layers 11, 12 project a suiiicient distance beyondthe longitudinal edges of foils 9, 10 to prevent short circuitingbetween the electrodes which, in opera= tion, are of opposite polarity.In electrical contact with the respective foil electrodes 9, 10 are tapstraps 13 and 14 which project from the ends of the roll. Tap strap 14is provided with a fuse device in accordance with the invention and aswill be seen by reference to FIGURE comprises tapered tap conductorportions 14a, 14b bridged by fuse link 15 with an electricallyinsulating sheet 16 enveloping the tap in the region of the use link andwith the margins of sheet 16 at its open side suitably joined together,e.g., by adhesive material. Metal foil sheet 17 is folded about theinner end of the tap for making effective contact between the latter andelectrode foil 10. Use of a flag such as sheet 17 in conjunction withtap 14 facilitates insertion of the tap into the roll during winding ofthe capacitor using automatic insertion equipment of known type.However, use of sheet 17 is not absolutely necessary, since tap 14 maybe secured at its lower portion 14b directly to foil 10 by any suitablemeans, such as by welding, stitching, etc. As shown in FIGURE 4, tap 14should be so arranged in roll 8 that portion 14a does not touch foil 10and to this end insulating cover sheet 16 should extend well beyond theedge of foil 10.

The tap-fuse arrangement shown in FIGURE 4 and particularly theprovision of cover sheet 16 are, as will be evident, necessary in theillustrated capacitor applica-' tion to prevent fuse link 15 from beingelectrically bypassed by the foil conductor which is in physical contactwith the tap. While it might ordinarily have been considered thatcovering the fuse link with an electrically insulating material such askraft paper would interfere with the desired thermal conditions in thefuse area, it has been found, however, that proper selection of theinsulating cover will result in a material which will not significantlyaffect the heat flow from the fuse device. By properly dimensioning thesurface area of insulating cover 16 and relating this factor to thethermal conductivity of its material, it is possible to provide abalance between the heat confined and the heat given off from theinsulating cover. Increased surface area of the insulating coverincreases thermal flow, while lower thermal conductivity of the covermaterial hinders heat. flow through it. The thickness of the sheet usedwill, of course, affect its thermal conductivity. In short, an insulating cover is used wherein the thermal conductivity of the sheetbalances its thermal resistance. The cover thus constitutes a thermalelement which is half way between a thermal conductor and a thermalinsulator and thereby contributes to a workable fuse dQV SZQ iQ I2tection of a roll capacitor.

FIGURE 6 illustrates the fused capacitor roll section 8 inclosed in asealed casing 18 containing a dielectric liquid 23 such as mineral oilor chlorinated diphenyl in which roll section 8 is immersed and whichimpregnates the dielectric sheets of the roll. Taps 13 and 14 areconnected respectively to external terminals 19 and 20 which passthrough insulating bushings 21, 22 mounted in cover 18a. The fuse deviceof the invention has been found to operate quite successfully in suchcapacitor assemblies.

By way of illustration, a capacitor fuse tap construction made inaccordance with the invention had a fusible element of 60% tin-40% leadforming a round wire of .020" diameter and .25" length, the fusibleelement being butt soldered to tapered ends of tinned copper taps havinga .160" width, .035" thickness and 2 /2" length. The fuse was coveredwith a gummed kraft paper sheet having unfolded dimensions of /2" width,.002" thickness and 1%" length.

The maximum continuous 60 cycle A.C. carrying current rating of theabove fuse construction in uncirculated 25 C. air was found to be 8.8amperes. The device functions in air, oil, or as part of a tap assemblyof a capacitor, with currents of the .order of 20% above sustainingcurrent levels blowing the fuse in less than five seconds, this beingachieved without overheating the capacitor or charring the cover of thefuse tap.

The above fuse (Sample A) which had a link with length/diameter ratio of12.5 to 1 was tested for performance characteristics with three otherfuses which had the identical construction except that one, Sample B,had a link length of .0125" (ratio of 6.25 to 1) and a current rating of10.5 amperes under the same conditions as with Sample A; another, SampleC, a link length of .0375 (ratio of 18.75 to 1) and current rating of6.5 amperes; and another, Sample D, a link length of .750" (ratio of37.5 to l), the latter being a typical length of fuse link used inconventional enclosed solder-wire link fuses, and having a currentrating of 4.5 amperes. Measured performance of these fuses showed thatthey blew in three seconds with the following percent values of currentload increase above continuous sustaining levels under the conditionsshown:

It will be evident from the above data that the Sample D length of fuselink which is substantially above the 20 to 1 ratio limit required aconsiderably greater increase in current load in order to blow in thesame time as the samples having length/diameter ratios within the rangesfound to be optimum in accordance with the invention.

While the present invention has been described with reference toparticular embodiments thereof, it will be understood that numerousmodifications may be made by those skilled in the art without actuallydeparting from the scope of the invention. Therefore, the appendedclaims are intended to cover all such equivalent variations as comewithin the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A fuse device comprising an elongated electrically conducting metalmember having a melting point not less than about 600 C. dividedintermediate its ends into two portions separated by a gap, and afusible member having a melting point not more than about 400 C.connected at its ends to said separated portions of said conductingmember and conductively bridging the gap therebetween, thecross-sectional area of said fusible member at each end thereof being atleast equal to the cross-sectional area of said metal conducting memberat the points of contact therewith, thereby insuring that the primarymode of heat dissipation is from said fusible member to the ambientrather than from said fusible member to said conducting member.

2. A fuse device comprising an elongated electrically conducting metalmember having a melting point not less than about 600 C. dividedintermediate its ends into two portions separated by a gap, and afusible member having a melting point not more than about 400 C.connected at its ends to said separated portions of said conductingmember and conductively bridging the gap therebetween, thecross-sectional area of said fusible member at each end thereof beingnot less than the cross-sectional area of said metal conducting memberat the points of contact therewith thereby insuring that the primarymode of heat dissipation is from said fusible member to the ambientrather than through said conducting member, and electrically insulatingmaterial covering said fusible member and being of such dimensions thatits thermal conductivity balances its thermal resistance.

3. A fuse device comprising an elongated electrically conducting metalmember having a melting point not less than about 600 C. dividedintermediate its ends into two portions separated by a gap, and afusible member having a melting point not more than about 400 C.connected at its ends to said separated portions of said conductingmember and conductively bridging the gap therebetween, thecross-sectional area of said fusible member being substantially circularand being approximately equal to the cross-sectional area of said metalconducting member at the points of contact therewith, the length of saidfusible member being not more than about twenty times its diameter,thereby minimizing the percent values of current load increase abovecontinuous sustaining levels before said fuse device blows.

4. A fuse device comprising an elongated strip-shaped electricallyconducting metal member having a melting point of not less than about600 C. and divided intermediate its ends into two portions separated bya gap, the ends of said portions adjacent the gap being tapered towardthe gap, and a fusible member having a melting point not more than about400 C. connected at its ends to the separated portions of saidconducting member and conductively bridging the gap therebetween, thefusible member having a circular cross-section and its cross-sectionalarea being approximately equal to the cross-sectional area of thenon-tapered portion of said metal conducting member at a point on theconducting member adjacent the tapered portion, the length of saidfusible member being in the range of about five to about twenty timesits diameter, thereby minimizing the percent values of current loadincrease above continuous sustaining levels before said fuse deviceblows.

5. A fuse device comprising an elongated electrically conducting metalmember having a melting point not less than about 600 C. and dividedintermediate its ends into two portions separated by a gap, a fusiblemember bridging said gap, said fusible member having a melting point nothigher than about 400 C., and solder material having a melting pointlower than that of said fusible member coating and joining the latter inelectrical contact with the ends of said conducting portions, thecross-sectional area of the fusible member at each end thereof being notless than the cross-sectional area of said metal member at the points ofcontact therewith, thereby insuring that the primary mode of heatdissipation is from said fusible member to the ambient rather thanthrough said conducting member.

(References on following page) 7 8 References Cited by the Examiner2,815,414 12/ 1957 Iwantscheff et a1 200136 2,983,856 5/1961 Martin eta1 317-260 UNITED STATES PATENTS 713,831 11/1902 Badeau 200 135 3 fjiPATENTS 776,660 12/1904 Glover 200-135 5 703,6 2 2 19 Germany- 12,704,341 3/1955 Stacy et a1. 317-256 JOHN F. BURNS, Primary Examiner.

1. A FUSE DEVICE COMPRISING AN ELONGATED ELECTRICALLY CONDUCTING METALMEMBER HAVING A MELTING POINT NOT LESS THAN ABOUT 600* C. DIVIDEDINTERMEDIATE ITS ENDS INTO TWO PORTIONS SEPARATED BY A GAP, AND AFUSIBLE MEMBER HAVING A MELTING POINT NOT MORE THAN ABOUT 400* C.CONNECTED AT ITS ENDS TO SAID SEPARATED PORTIONS OF SAID CONDUCTINGMEMBER AND CONDUCTIVELY BRIDGING THE GAP THEREBETWEEN THECROSS-SECTIONAL AREA OF SAID FUSIBLE MEMBER AT EACH END THEREOF BEING ATLEAST EQUAL TO THE CROSS-SECTIONAL AREA OF SAID METAL CONDUCTING MEMBERAT THE POINTS OF CONTACT THEREWITH, THEREBY INSURING THAT THE PRIMARYMODE OF HEAT DISSIPATION IS FROM SAID FUSIBLE MEMBER TO THE AMBIENTRATHER THAN FROM SAID FUSIBLE MEMBER TO SAID CONDUCTING MEMBER.